The invention relates to a method of operating a projection exposure tool for microlithography, a control apparatus for operating such a projection exposure tool and a projection exposure tool including such a control apparatus.
For an economical operation of a projection exposure tool, the shortest possible exposure time is desirable, so as to achieve a high throughput rate of substrates being exposed. Therefore high radiation intensities are required for an adequate exposure of the substrates. In particular, when using radiation in the ultraviolet or extreme ultraviolet (EUV) wavelength region, the influence of intensive radiation can result in radiation-induced changes in the properties of the optical elements of the projection exposure apparatus, and the magnitude of these changes increase with the radiation dose. For example, absorbed radiation in a lens can have the effect of a localized temperature change in the lens, which may result in a localized change of refractive index and surface of the lens. This effect is a reversible, short-term effect and known as “lens heating”. Further, radiation can result in irreversible, long-term effects in the lens known as “life-time effects”. Such effects include compaction (increased density) or rarefaction (reduced density) of the lens. When using 193 nm radiation a particularly large radiation-induced density increase is observed in quartz glass (fused silica).
According to a known method a so called “full chip” simulation is performed based on the layout of the whole mask to be imaged onto a wafer using the so-called fininte element method (FEM) to calculate the temperature distribution within the lens. This rigorous approach requires large computing resources and is too time-consuming to correct short term radiation-induced effects on the lithographic image based on this method.